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Fri, 30 Oct 2026 22:24:54 -0700 (PDT) Received: from asahi.tail4da9e3.ts.net ([218.48.198.193]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-36a428af91bsm12891099eec.8.2026.10.30.22.24.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 30 Oct 2026 22:24:54 -0700 (PDT) From: "Baek Kyu, Kim" To: Balbir Singh Cc: "Baek Kyu, Kim" , Andrew Morton , Oleg Nesterov , linux-kernel@vger.kernel.org Subject: [PATCH] taskstats: Avoid counting an exiting thread twice in TGID queries Date: Sat, 31 Oct 2026 14:24:51 +0900 Message-ID: <20261031052451.78846-1-pocj8ur4in@gmail.com> X-Mailer: git-send-email 2.55.0 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit fill_tgid_exit() adds a thread's contribution to signal->stats before exit_notify() sets exit_state. A TGID query in that interval copies the cached contribution, then scans the same thread because exit_state is still clear. The thread's contribution is counted twice. Mark a task when its contribution reaches the cache, under the same siglock used by the TGID query. Exclude marked tasks from the live scan. Leave a task eligible for the scan if no cache exists, and clear the marker when a new task is copied during fork. Fixes: ad4ecbcba728 ("[PATCH] delay accounting taskstats interface send tgid once") Signed-off-by: Baek Kyu, Kim --- In an arm64 QEMU guest, holding the interval after cache accounting but before exit_state changed made a 100 ms worker appear twice in a TGID CPU query. The baseline reported 208000 us for 8000 us of the querying thread plus 100000 us of the worker; the change reported 112000 us for 12000 us plus 100000 us. Natural race frequency and other architectures were not measured; KASAN and KCSAN were not enabled. include/linux/sched.h | 5 +++++ kernel/fork.c | 3 +++ kernel/taskstats.c | 7 ++++--- 3 files changed, 12 insertions(+), 3 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index d35ae49a991f..359ee982323c 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1457,6 +1457,11 @@ struct task_struct { struct task_delay_info *delays; #endif +#ifdef CONFIG_TASKSTATS + /* Protected by sighand->siglock. */ + bool taskstats_tgid_accounted; +#endif + #ifdef CONFIG_FAULT_INJECTION int make_it_fail; unsigned int fail_nth; diff --git a/kernel/fork.c b/kernel/fork.c index 33760ece3bb3..76d9abcc5ca8 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2202,6 +2202,9 @@ __latent_entropy struct task_struct *copy_process( goto bad_fork_cleanup_count; delayacct_tsk_init(p); /* Must remain after dup_task_struct() */ +#ifdef CONFIG_TASKSTATS + p->taskstats_tgid_accounted = false; +#endif p->flags &= ~(PF_SUPERPRIV | PF_WQ_WORKER | PF_IDLE | PF_NO_SETAFFINITY); p->flags |= PF_FORKNOEXEC; INIT_LIST_HEAD(&p->children); diff --git a/kernel/taskstats.c b/kernel/taskstats.c index f31df72f0e9d..dd9054de761f 100644 --- a/kernel/taskstats.c +++ b/kernel/taskstats.c @@ -245,8 +245,8 @@ static int fill_stats_for_tgid(pid_t tgid, struct taskstats *stats) u64 now_ns; /* - * Add additional stats from live tasks except zombie thread group - * leaders who are already counted with the dead tasks + * Add stats from tasks not yet included in the dead-task cache. + * Accounting can finish before exit_state is set. */ rcu_read_lock(); first = find_task_by_vpid(tgid); @@ -261,7 +261,7 @@ static int fill_stats_for_tgid(pid_t tgid, struct taskstats *stats) now_ns = ktime_get_ns(); for_each_thread(first, tsk) { - if (tsk->exit_state) + if (tsk->exit_state || tsk->taskstats_tgid_accounted) continue; tgid_stats_add_task(stats, tsk, now_ns); @@ -291,6 +291,7 @@ static void fill_tgid_exit(struct task_struct *tsk) now_ns = ktime_get_ns(); tgid_stats_add_task(tsk->signal->stats, tsk, now_ns); + tsk->taskstats_tgid_accounted = true; ret: spin_unlock_irqrestore(&tsk->sighand->siglock, flags); return; -- 2.55.0